Mercury and other heavy metals are contained in addition to nitrogen oxides and sulfur oxides in an exhaust gas discharged from a thermal power plant boiler, etc., which is a combustion apparatus that uses coal or other fossil fuel. Although the nitrogen oxides are removed by NOx removal equipment and the sulfur oxides are removed by a desulfurizer, mercury, selenium, cadmium, chromium, lead; zinc, and other heavy metals cannot be removed by the NOx removal equipment or desulfurizer and cannot be trapped completely by a precipitator for removing soot/dust in the exhaust gas. Because these heavy metals are high in toxicity, emission restrictions thereof have recently become stricter. Methods for removing heavy metals contained in the exhaust gas, particularly mercury, which is high in toxicity, are being examined.
FIG. 8 illustrates one such method and shows an exhaust gas treating system applied to remove mercury in an exhaust gas from a waste incinerator or other combustion apparatus 1. With this exhaust gas treating system, the exhaust gas from the combustion apparatus 1 is first treated by NOx removal equipment 2 to remove nitrogen oxides, and combustion air to be used in the combustion apparatus 1 is then heated by the exhaust gas in an air preheater 3. Soot/dust in the exhaust gas discharged from the air preheater 3 are then trapped by a precipitator 4, the exhaust gas discharged from the precipitator 4 is guided by an induction fan 5 to a wet flue gas desulfurizer 6 to remove sulfur oxides in the exhaust gas, and the exhaust gas is discharged by a boost-up fan 7 into air from a chimney 8. A measurement unit 9 for measuring concentrations of the sulfur oxides, nitrogen oxides, heavy metals (Hg), etc., in the exhaust gas is disposed at an upstream side of the chimney 8 to monitor concentrations of these components in the exhaust gas.
The exhaust gas treating system shown in FIG. 8 is characterized in that a mercury adsorbent is added by a mercury adsorbent supply unit 16, etc., into the exhaust gas at an entrance (front stage) of the precipitator 4, and with this exhaust gas treating system, the exhaust gas at the entrance (front stage) of the precipitator 4 is in a temperature range (150 to 240° C.) in which a mercury adsorption performance of activated carbon, etc., is highest and the activated carbon, etc., to which mercury has become adsorbed, is recovered by the precipitator 4 (see, for example, Non-Patent Document 1, etc.).
FIG. 9 shows an exhaust gas treating system applied to removing mercury in an exhaust gas from a thermal power plant boiler, and with this system, heavy metal concentrations in the exhaust gas that is discharged to the atmosphere are adjusted to be within predetermined ranges by lowering an exhaust gas temperature to economically remove heavy metals in the exhaust gas without using an absorbing agent for absorbing the heavy metals in the exhaust gas.
With the exhaust gas treating system shown in FIG. 9, the exhaust gas from the boiler or other combustion apparatus 1 is supplied to the NOx removal equipment 2 to be removed of the nitrogen oxides and is thereafter used for heating the combustion air in the air preheater 3. A heating medium in a heat recovery unit 11 is then heated by the exhaust gas discharged from the air preheater 3, the soot/dust in the exhaust gas that has been lowered in temperature and is discharged from the heat recovery unit 11 are trapped efficiently by the precipitator 4, and the exhaust gas discharged from the precipitator 4 is guided by the induction fan 5 to the wet flue gas desulfurizer 6 to be desulfurized. The exhaust gas discharged from the wet flue gas desulfurizer 6 is reheated by a reheater 13 using the heating medium that is circulatingly supplied from the heat recovery unit 11 via heating medium circulating ducts 15-1 and 15-2, and is discharged by the boost-up fan 7 into air from the chimney 8 (Patent Document 1). The measurement unit 9 for measuring the concentrations of the sulfur oxides, nitrogen oxides, heavy metals, etc., in the exhaust gas before discharge from the chimney 8 is also disposed in the system shown in FIG. 9 to monitor the concentrations of these components in the exhaust gas.
The exhaust gas treating system shown in FIG. 9 makes use of the fact that the heavy metals in the exhaust gas become attached more readily to solid surfaces of ash particles, etc., the lower the exhaust gas temperature, and the heavy metals in the ash particles can be recovered by an appropriate method or a treatment for preventing elution from the ash particles can be applied. In Patent Document 1, it is disclosed that a heavy metal concentration in the exhaust gas can be suppressed by a method of measuring the concentration of the heavy metal (Hg) in the exhaust gas discharged from the wet flue gas desulfurizer 6 in the exhaust gas treating system shown in FIG. 9 and adjusting one or more among: a pH of an absorbing solution used in the wet flue gas desulfurizer 6; an oxidizing air flow rate; and a wastewater flow rate; so that a measurement value of the heavy metal concentration falls within a predetermined range.
Non-Patent Document 1: Tatsuhiro Fujii and six others, “Technology on the Bag Filter System for Flue Gas Treatment to Remove Simultaneously Many Kinds of Harmful Compounds from a Municipal Solid Waste Incinerator,” Hitachi Zosen Technical Review, Hitachi Shipbuilding Corporation, June, 1992, Vol. 53, No. 2, p. 23-30. Patent Document 1: International Patent Publication No. 2004/023040 Pamphlet